EP 0 712 007 B1 discloses a battery-operated razor with a charging state indicator and a device that detects a charging state, at which the battery is discharged to such a degree that the razor can only continue to operate for a very short time. When this charging state is reached, a pulse-width modulator supplies the DC motor of the razor with a pulsating voltage of constant frequency, the pulse-duty factor of which is gradually reduced to zero such that the motor speed is reduced accordingly. This makes a user aware of the fact that the motor and therefore also the cutting tool driven by the motor will soon stop such that the user will intuitively stop shaving before beard hairs can get caught in the stopped cutting tool.
There also exist battery-operated razors and electric toothbrushes that are driven by an oscillating electric motor or linear motor. These motors have a resonant frequency, with which they generally need to be operated in order to achieve an adequate efficiency. When the battery voltage drops as the battery is increasingly discharged, these motors continue to run with their resonant frequency at a reduced amplitude. Consequently, a user is unable to determine that the motor will soon stop due to a discharged battery based on the motor noise. The user may misinterpret this sudden stop as a defect, wherein the sudden stop of a razor may lead to beard hairs getting caught in the razor and the pain associated therewith. Thus, attempts have been made to make the user aware of the empty battery and the impending stop of the motor in a timely fashion with a beeping and blinking indicator.